Hormone replacement therapy (HRT) has been shown to reduce some of the long term consequences of aging such as maintaining bone density and lowering the risk of cardiovascular disease (CVD). Because of the well-established increased risk of endometrial cancer associated with estrogen replacement therapy, there has been an increasing prescription of progestins in addition to estrogen for at least part of the cycle. There is some sparse evidence that although such combined HRT may reduce the risk of endomeuw cancer, it may no longer protect against CVD, and, because of the monthly bleeding associated with this therapy, may not be as acceptable to post-menopausal women. Consequently some experts are advocating low dose progestins, and others are advocating continuous addition of progestins in the hopes of avoiding one or both of these problems. Unfortunately there is some preliminary evidence from our prior study that neither of these regimens confer the same protective effect on endometrial cancer. It is vital that quantification of this risk for different estrogen-progestin regimens be available. The proposed study will: 1. Estimate the relative risks of endometrial cancer in postmenopausal women associated with different numbers of days in the cycle on which progestins are added, particularly continuous regimens, in combined estrogen plus progestogen therapy, relative both to no therapy, and to estrogen only therapy. 2. Estimate the relative risks of endometrial cancer associated with different doses of added progestin in combined HRT, particularly low dose regimens, relative both to no therapy, and to estrogen only therapy. 3. Estimate the relative risks of endometrial cancer associated with each schedule (cyclic or continuous) of and dose of added progestin compared both to no therapy and estrogen only therapy. The cases for this study are women aged 45-74 years, residing in King, Pierce or Snohomish counfies, Washington, U.S.A., who were diagnosed with endometrial cancer between January 1991 and December 1994, as identified through a population-based cancer registry. All potential endometrial cancer cases will be subjected to independent histological review. It is anticipated that approximately 617 incident endometrial carcinoma cases will be enrolled. About 430 controls will be recruited through random digit dialling. These will be combined with data from a prior study to yield a total of 1234 cases and 1222 controls for analysis. Cases and controls will be compared on history of past hormone use, taking into account potential confounding factors such as age, race, parity, and obesity.